This invention pertains generally to the field of micromachined parts and actuators and to the assembly of micromachines.
Various stiction phenomena cause major problems in micromachined devices because of the large surface to volume ratio of micromachined parts. As such parts are released from the underlying substrate, liquid forces can pull the devices to the surface of the substrate, where they may be permanently stuck. Several methods for releasing such microstructures or preventing bonding to the substrate in the first place have been developed, including dry release and the use of self-assembled monolayers. See, e.g., R. Maboudian, et al., xe2x80x9cStiction Reduction Processes for Surface Micromachines,xe2x80x9d Tribology Letters, Vol. 3, 1997, pp. 215-221.
In addition to the stiction or surface bonding of parts that occurs during forming of the micromachined parts, an additional significant problem is in-use stiction, wherein previously released microparts become stuck to the substrate or other parts. For example, high voltages are often required for speed and sensitivity in micromachined devices, and such voltages can pull the previously released part to the surface where it then becomes stuck. Various methods have been developed to overcome this sticking or bonding problem during use, including providing the part with self-assembled monolayers and Lorentz force approaches. The Lorentz force has been used to release stuck parts, but this method requires an external magnetic field. See, B. Gogoi, et al., xe2x80x9cAdhesion Release and Yield Enhancement of Microstructures Using Pulsed Lorentz Forces,xe2x80x9d JMEMS, Vol. 4, 1995, pp. 185-192. While the use of surface assembled monolayers can reduce the stiction problem that is encountered during use of the microparts, such monolayers on the parts can degrade the device performance. In addition, the monolayers wear out over time. Thus, the re-release of stuck micromachined parts during use continues to be a major concern for micromachined devices.
The assembly of large arrays of surface micromachined parts is another challenge in the commercialization of microelectrical-mechanical systems (MEMS) technology. Series assembly or erection of parts is too slow and costly. On-chip robots have been used, but consume valuable surface area. See, L. S. Fan, et al., xe2x80x9cSelf-Assembled Microactuated XYZ Stages for Optical Scanning and Alignment,xe2x80x9d Transducers 1997, Chicago, Ill., 1997. Ultrasonic vibration has been used to overcome friction and to shake parts around for assembly. M. Cohn, et al., xe2x80x9cParallel Microassembly with Electrostatic Force Field,xe2x80x9d Proc. IEEE International Conf. on Robotics and Automation, Vol. 2, 1998, pp. 1204-1211. Liquid forces have been used for assembling GaAs lasers on silicon dies. H. J. Yeh, et al., xe2x80x9cFluidic Self Assembly of Microstructures and its Application to the Integration of GaAs on Si,xe2x80x9d IEEE International Workshop on MEMS, Oiso, Japan, 1994, pp. 279-284. However, the ultrasonic vibration and liquid assembly approaches offer little directional control. In addition, the use of liquid forces may not be suitable for surface micromachined parts due to the effects of surface tension and stiction.
In accordance with the invention, microparts can be freed from a surface of a substrate to which the micropart is stiction bonded in an efficient and economical manner. Microparts can be freed from their underlying substrates to overcome stiction bonding forces incurred during production of the devices, as well as to free microparts that have become incidentally bonded to the underlying substrate after initial freeing of the part from the substrate. It is a particular advantage of the present invention that it can be carried out after the micromachined devices are fully formed, assembled, and packaged, at any time during the useful life of the devices.
In carrying out the invention, microparts, such as gears, beams, sliding actuators, etc., are formed on or assembled on the top surface of the substrate that is typically but not necessarily single crystal silicon. A transducer is secured to the substrate, preferably at the bottom surface of the substrate which is opposite to the top surface with which the microparts are in contact. A preferred transducer is a piezoelectric transducer which can be secured to the bottom surface of the substrate in various ways, such as by adhesive, mechanical pressure contact, or by being integrally formed with the substrate. To free the microparts from the surface to which they are stiction bonded, a voltage pulse is applied to the piezoelectric transducer which deforms in response thereto to apply a pulse stress wave to the substrate. The pulse stress wave propagates through the substrate and is reflected at the surface to which the micropart is stiction bonded. The reflection of the pulse stress wave at the top surface causes a rapid up and down displacement of that surface and a spalling action at the surface which breaks the bond between the micropart and the substrate surface. The substrate may comprise, for example, a semiconductor wafer such as single crystal silicon, on which electronic components may also be formed.
Further, in accordance with the invention, microparts that are in contact with but not bonded to a substrate surface may be displaced from that surface by applying a pulse stress wave to the substrate. The pulse stress wave propagates through the substrate to the top surface, where it is reflected with a rapid displacement of the top surface toward and then away from the micropart to transfer energy from the reflected pulse stress wave to the micropart, thereby to displace the micropart away from the surface by the impact energy transferred from the reflected stress wave. Microparts may be displaced from a first position, in which they are in contact with the surface, to a second stable position in which they are displaced from the surface. In particular, micropart panels may be connected by a hinge secured to the top surface of the substrate such that the impact to the panel from the displacement of the top surface causes the panel to rotate about the hinge from a first position resting against the top surface to a second position in which the panel is partially or fully upright and in which it is held stable. The panel may be formed with one or both of its surfaces reflective so that it will reflect a beam of light when it is in its upright position, and pass the beam of light when it is in its first position in contact with the surface. An array of such panels connected by hinges to the top surface may be provided and may be selectively maintained in their first or second positions to provide a desired steering of a light beam through the array. Other micropart components which require displacement from the surface to transfer the microparts to their erected positions can be so displaced utilizing the present invention.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.